[unreadable] [unreadable] Physical activity reduces the risk of cancers of the colon, breast, and endometrium. Several mechanisms that explain these links have been studied, including potential cancer preventative effects of exercise on inflammation, insulin-like growth factors, insulin resistance, steroid hormones, and lipid metabolism. Yet these mechanisms do not fully explain the links between exercise and cancer. Less explored is the effect of exercise on oxidative stress and lipid peroxidation; although recent evidence suggests: (1) a specific marker of lipid peroxidation, 8-iso-prostaglandin F2 (8-iso-PGF2a), is linked with breast cancer; and (2) in laboratory animals and short-term trials of men, 8-iso-PGF2a is decreased by exercise or diet and exercise combined. [unreadable] [unreadable] The primary aim of the proposed study is to assess the impact of a 1-year moderate-intensity exercise intervention (n =87) compared to a 1-year stretching control program (n = 86) on urinary 8-iso-PGF2a from a recently completed randomized controlled exercise intervention trial. The effect of the exercise intervention will be assessed on an intention-to-treat basis, using generalized estimating equations that account for within subject correlation of 8-iso-PGF2a over repeated measurements. Participants were postmenopausal women, aged 50-75 years, not taking exogenous hormones, overweight or obese, non-smokers, willing to be randomized, and residing in the greater Seattle area. Women in the exercise intervention arm were prescribed at least 45 minutes/day of moderate-intensity activity, 5 days/week. The randomized controlled trial had excellent retention; 98% of the participants completed the trial. Adherence to the exercise prescription was high as well; women in the exercise arm participated in sport or recreational activity an average of 3.7 [unreadable] 1.4 days/week for 171 [unreadable] 88 min/week (87% of prescribed activity). As secondary aims we will evaluate baseline and longitudinal correlations between 8-iso-PGF2a and previously collected data from this study, including a measures of adiposity (e.g. weight, height, subcutaneous and intra-abdominal fat surface area, percent body fat, total body fat, and waist and hip circumferences), inflammatory markers (e.g. C-reactive protein, serum amyloid A, and interleukin-6), use of dietary supplements, and aerobic fitness. In summary, this application builds on existing high quality data from a previously established randomized controlled trial to examine the effect of a 1-year of moderate-intensity exercise program on a novel biomarker of cancer risk. [unreadable] [unreadable] [unreadable] [unreadable]